I found the source code from a freebsd ftp by I'm not sure it's the code that the OSX version. Also I check apple source code web site but did not find it.
ipfw is inherited from FreeBSD; you can find current FreeBSD ipfw source here: http://www.freebsd.org/cgi/cvsweb.cgi/src/sbin/ipfw/, but OS X's ipfw source is here: http://www.opensource.apple.com/source/network_cmds/network_cmds-329.2.2/.